In this lesson, students will learn how to find the area of a parallelogram. Through student discussion and discovery, students will understand the relationship between the area of a rectangle and parallelogram. Students will also start using vocabulary like “base” and “height” when finding the area. You can expect this lesson with additional practice to take one `45`-minute class period.
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The warm-up will be an essential part of this lesson to help introduce students to finding the area of a parallelogram.
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Students will have various different ways of describing these two figures. Capture all the vocabulary words that they use. Some might wonder what the markings on the side mean. Some might misuse the vocabulary words. This warm-up will give you a good sense of students’ current understanding of parallelograms.
Introduce the name of this new shape: parallelograms. I find it normally helps to informally explain a parallelogram as a “slanted rectangle”. Since this is students’ first experience with parallelograms, you’ll want to explain the key features. The key features you’ll want to bring up include:
Because the parallelogram on the right shows the grid within the figure, students may have different approaches to finding the area of the parallelogram. Giving students time to think about their own approaches and encourage students to share their ideas with each other. Some students will find the area by counting how many of the boxes are cut in half so they can try to make whole boxes. Then they count how many boxes there would be in all. While the previous method does work, there will likely be some students who recognize that the left side of the parallelogram could be moved to create a rectangle. Then, they just have to find the area of the rectangle by counting the length and width.
As you listen to student conversations, try to note which students use the method of rearranging the parallelogram. Then, when reviewing as a class, make sure at least one of these students is able to share their method and why it works. This is an essential part of the lesson because it allows students to see how a parallelogram can become a rectangle. It also helps connect the formula for the area of a rectangle to the area of a parallelogram. One great way to transition is to ask students if we can still use a formula like “Area `=` length `\times` width” if the figure is not a rectangle. This is a great opportunity for you to introduce the vocabulary of “base” and “height” for the parallelogram, and some students may even be able to guess what the different words should be.
Share this slide with students so that they can copy it for reference.
Students should try to fill in the blanks if they are able to. It is important that students understand the base is always a side of the parallelogram. Students should also understand that the height always makes a right angle with the base. Keep in mind that the focus of this slide is to help students understand how to identify the base and height in order to help them find the area of a parallelogram.
For each figure, see if students can identify which value is the base and which value is the height. You could have students vote by raising their hands or giving a thumbs up for each number that is possible. When reviewing their answers, it may be helpful to pose some additional questions to them, such as:
The next three examples are important for helping students practice finding the area of a parallelogram. The first example requires students to identify the base and height before calculating the area, the second is a rotated parallelogram with decimals, and in the third, they are given a verbal description.
Start with this simpler example where the orientation of the parallelogram is the way a student would expect. Have students identify the base and height first, then multiply to find the area.
This next example gets a bit trickier for students, since it has a different orientation. Help students to identify the base and height by reminding them that the base is always a side length of the parallelogram. You may also need to help students through the calculations with decimals.
To further practice with decimals, students can try this problem on their own or with a partner. Again, students will need to be careful of their decimals. Some students may find this problem easier because the information is given to them directly; however, it gives the students an opportunity to see how to find the area of a parallelogram even if an image is not given.
After you’ve completed the examples with the whole class, it’s time for some independent practice! ByteLearn gives you access to tons of mild, medium, and spicy practice problems for finding the area of a parallelogram. Check out the online practice and assign to your students for classwork and/or homework!
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